SANTIAGO, Chile -- Smoke from dozens of nearby forest fires blanketed the Chilean capital on Wednesday, leading authorities to issue a health alert.

The fast-spreading fires reached a peak over the weekend. They have caused more than $100 million in property losses and forced the seven million residents of Santiago to breathe the worst air in 15 years.

On Wednesday, strong winds carried a thick blanket of smoke over Santiago, where 40 percent of Chileans live. Firefighters have been struggling with the blazes as the wind has reignited several of them.

President Sebastian Pinera announced the health alert for four regions at a press conference late Wednesday. Pinera said the more than 70 forest fires over the past weeks have affected more than 40,000 hectares (100,000 acres) of land.

The emergency measure will be administered by Chile's health minister. It grants special powers to protect the health of those affected by the blazes, including ordering extra masks and oxygen tanks or the hiring of more ambulances and medical staff. Pinera also said the alert was issued to curb the spread of diseases transmitted by rodents, which have been fleeing the fires.

Earlier on Wednesday, the Santiago government had warned residents about the poor air quality and asked them to avoid exercising outdoors.

"We could see very high levels of contamination," said Deputy Health Secretary Jorge Diaz.

Joggers and bikers exercising at the San Cristobal hill in downtown Santiago began to return home as the smoke cloud arrived Wednesday afternoon.